The invention relates to a process for obtaining a compressed product by low temperature separation of air in a rectification system which has a pressure column (high pressure column) and a low pressure column, this process comprising the following steps:
a. a first flow of compressed and purified feedstock air is cooled in a main heat exchanger system and is fed into the pressure column,
b. at least one fraction from the pressure column is expanded and fed into the low pressure column,
c. an oxygen-rich fraction from the low pressure column is liquid-pressurized and delivered to the mixing column,
d. a heat exchange medium is fed into the lower area of the mixing column and is brought into countercurrent contact with the oxygen-rich fraction,
e. a gaseous top product is removed from the upper area of the mixing column and
f. a product fraction is removed from the rectification system, liquid-pressurized, vaporized in indirect heat exchange with the gaseous top product of the mixing column and is withdrawn as the compressed product, characterized in that
g. indirect heat exchange is carried out for vaporization of the liquid-pressurized product fraction in the main heat exchanger system.
The rectification system of the invention can be made as a classical double column system, but also as a three-column or multicolumn system. In addition to the columns for nitrogen-oxygen separation, it can have additional devices for obtaining other air components, especially rare gases. In addition to the rectification system, in the process a mixing column is used in which an oxygen-rich fraction is vaporized from rectification in direct heat exchange with a heat exchange medium. The top gas of the mixing column is used for indirect vaporization of a liquid-pressurized product fraction (so-called internal compression).
The oxygen-rich fraction which is used as the feedstock for the mixing column has an oxygen concentration which is higher than that of air and is for example 70 to 99.5% by mole, preferably 90 to 98% by mole. A mixing column is defined as a countercurrent contact column in which a more easily volatile gaseous fraction is sent opposite a more poorly volatile liquid.
The process of the invention is suitable for obtaining gaseous compressed oxygen and/or gaseous compressed nitrogen, especially for producing gaseous impure oxygen under pressure. Here impure oxygen is defined as a mixture with an oxygen content of 99.5% by mole or less, especially from 70 to 99.5% by mole. The product pressures are for example 3 to 25 bar, preferably 4 to 16 bar. Of course the compressed product if necessary can be further compressed in the gaseous state.
A process of the initially mentioned type is known from DE 19803437 A1. Here liquid oxygen is pumped and vaporized in the top condenser of the mixing column.
The object of the invention is to make the initially mentioned process economically more favorable, especially by hardware simplification and/or energy saving.
This object is achieved in that indirect heat exchange for vaporization of the liquid-pressurized product fraction is no longer done in a separate condenser-evaporator, but in the main heat exchanger system in which the pressure column air is also cooled. Preferably the product fraction is introduced immediately after pressurization rise (for example, in a pump) into the cold end of the main heat exchanger system, there first heated to the boiling point and then vaporized, both against the condensing or condensed top fraction of the mixing column.
In this way a separate condenser-evaporator which is necessary in the process from DE 19803437 A1 can be eliminated, as can a separate heat exchanger for removing the supercooling from the liquid-pressurized product fraction. By integrating the vaporization of the liquid product fraction and the cooling of air moreover the heat exchange process (Q-T diagram) can be improved so that especially small exchange losses are achieved and thus relatively low energy consumption is achieved.
The main heat exchanger system in the sense of this invention can, but need not, be implemented by a single heat exchanger block. It can also consist of several blocks connected in parallel or series. With parallel connection the blocks have the same inlet and outlet temperatures. Generally vaporization and at least part of the heating of the liquid-pressurized product flow take place in the same heat exchanger block.
The mixing column is operated under a pressure which is enough to vaporize the product fraction below the desired pressure against the condensing top gas of the mixing column, for example below 5 to 17 bar, preferably below 5 to 13 bar. The pressure of the high pressure column in the invention is in the range of for example 5 to 15 bar, preferably 5 to 12 bar, that of the low pressure column for example 1.3 to 6 bar, preferably 1.3 to 4 bar.
Preferably the top product of the mixing column downstream of the condensation which takes place in the condenser-evaporator is expanded and recycled into the low pressure column. The top product is introduced therein at a feedpoint, above by at least one theoretical plate (for example, one to ten theoretical plates) the removal point of the oxygen-rich fraction. Between the condenser-evaporator and expansion, the fluid is optionally cooled, for example by indirect heat exchange with the product fraction and/or the oxygen-rich fraction.
Preferably a second flow of purified feedstock air is compressed to a pressure which is clearly higher than the operating pressure of the pressure column, is cooled in the main heat exchanger system, and then fed into the mixing column as a heat exchange medium. This second air flow at the same time delivers at least some of the heat for heating the liquid-pressurized product fraction downstream of its vaporization. xe2x80x9cClearly higherxe2x80x9d is defined here as a pressure difference which is higher than the line losses, especially higher than 1 bar. This pressure difference can be achieved for example by all the air being compressed essentially to the pressure column pressure and then its being branched into two air flows, the second flow being further compressed, for example by a motor-driven compressor. Alternatively, the two air flows can be compressed separately from the atmospheric pressure to the pressures required at the time. The pressure to which the second air flow is compressed is generally 1.1 to 2.0 times the pressure of the liquid product fraction during its vaporization.
It is furthermore favorable when the second flow after its cooling in the main heat exchanger system and before it is fed into the mixing column is further cooled in indirect heat exchange with the liquid-pressurized oxygen-rich fraction. Thus the two feedstock fractions of the mixing column are brought to the temperature which is optimum for their feed.
For optimization of the Q-T diagram of the main heat exchanger system it is advantageous if the second flow at a first intermediate point below a first intermediate temperature is removed from the main heat exchanger system, the first intermediate temperature being clearly higher than its dew point. The gaseous top product of the mixing column is introduced into the main heat exchanger system at the first intermediate point at which the second flow is removed from the main heat exchanger system. In this way the same passage in the main heat exchanger system can be used both for cooling of the second air flow and also for condensation of the top product of the mixing column.
If the compressed product is oxygen, the product fraction is removed from the low pressure column. The product fraction and the oxygen-rich fraction for the mixing column can then be jointly withdrawn from the low pressure column and/or jointly liquid-pressurized; in hardware terms this is especially simple. Alternatively, the product fraction and the oxygen-rich fraction can be removed at different points of the low pressure column. The oxygen-rich fraction is preferably withdrawn at least one theoretical or practical plate above the removal point of the product fraction from the low pressure column.
Alternatively or in addition to the compressed oxygen, nitrogen can be obtained as the compressed product. The (additional) product fraction is then removed from the pressure column, if necessary for example liquefied in the top condenser of the pressure column, liquid-pressurized separately from the oxygen-rich fraction and vaporized and heated in the main heat exchanger system.
In the lower area a liquid fraction, for example the bottom liquid, is removed from the mixing column, expanded and delivered to the pressure column or to the low pressure column. In the case of feed into the low pressure column, the feed point is preferably above the removal of the oxygen-rich fraction and the return feed of the top fraction from the mixing column, preferably one to twenty theoretical plates above the introduction of the return feed of the top fraction to the mixing column. Before expansion, the liquid fraction from the mixing column is optionally cooled, for example by indirect heat exchange with the product fraction and/or the oxygen-rich fraction.
The invention relates moreover to a device for obtaining a compressed product by low-temperature separation of air system which has a pressure column (3) and a low pressure column (4)
a. with a first feedstock air line for feeding compressed and purified feedstock air via the main heat exchanger system into the pressure column,
b. with a liquid transfer line for feed of a fraction from the pressure column into the low pressure column, the liquid transfer line having an expansion means,
c. with a means for increasing the pressure of the oxygen-rich fraction from the low pressure column with an outlet which is flow-connected to the mixing column,
d. with a supply line for feeding the heat exchange medium into the lower area of the mixing column,
e. with a top product line for removing the gaseous top product from the upper area of the mixing column,
f. with means for increasing the pressure of a liquid product fraction from the rectification system with an outlet which is flow-connected to the product evaporator which is also connected to the head product line and to the compressed product line
wherein
g. the product evaporator is formed by the main heat exchanger system.
The invention and further details of the invention are explained below using the embodiments shown schematically in the drawings.